A power module, which is used for driving a motor or the like, includes a circuit board on which a semiconductor device (chip) (e.g., a power transistor) and a heat spreader (heat sink member) are mounted. Recently, semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors) which are capable of rapid operation are mainly used.
With reference to FIG. 8, the schematic outline of a generic power module will be described.
A power module 300 is composed of a heat radiating member 101, a circuit board 108 such as a ceramic substrate, and a semiconductor chip 109 such as an IGBT. The circuit board 108 is a Direct Copper Bonding substrate, in which copper-foil circuit boards 108b and 108c are directly bonded onto both faces of a ceramic plate 108a that is composed of alumina, aluminum nitride, silicon nitride or the like. A solder layer 112 such as Sn—Pb is used for bonding between the heat radiating member 101 and the circuit board 108. A solder layer 111 such as Ag—Cu is used for bonding between the circuit board 108 and the semiconductor chip 109.
In recent years, as circuits become more and more highly-integrated and as semiconductor devices improve in operating speed, the power consumption of semiconductor chips is greatly increasing, and the amount of heat generated by chips is also rapidly increasing. Heat generation of a chip not only detracts from the operating speed and lifespan of a device, but also causes considerable problems of chip peeling and breaking.
In order to solve this problem, a material used for a heat spreader is required to have a high thermal conductivity as well as a coefficient of thermal expansion which is substantially equal to the coefficient of thermal expansion of the semiconductor chip. The reason is that, if there is a large difference between the coefficient of thermal expansion of the material of the heat spreader and the coefficient of thermal expansion of the semiconductor chip, the semiconductor chip may peel from the heat spreader or break, no matter how good a thermal conductivity the material may have.
Conventionally, as heat spreaders, composite materials each composed of different kinds of metals are generally used, e.g., Cu—Mo substrates and Cu—W substrates. Such substrates are composed of Cu having a high thermal conductivity and Mo or W, whose coefficient of thermal expansion only has a small difference from that of a semiconductor device of Si or the like, and therefore they exhibit practically satisfactory values in terms of both thermal conductivity and coefficient of thermal expansion. In particular, Cu—Mo substrates are generally used because Mo is less expensive than W. As Cu—Mo substrates, for example, Cu—Mo clad composites, in each of which a Cu base and an Mo base are bonded via rolling or the like, are generally used.
As mentioned above, a heat spreader is bonded to a circuit board or a semiconductor device via brazing. Since Cu and Mo differ in wettability and the like with respect to the brazing material, the surface of a Cu—Mo substrate is usually covered with an Ni plating layer, with the purpose of facilitating brazing and enhancing anticorrosiveness.
However, Cu and Mo are quite different in their abilities to allow an Ni plating layer to be formed thereon. Therefore, within one plating bath, it is difficult to form Ni plating layers showing excellent adhesion both on the surface of the Cu base and on the surface of the Mo base at the same time. As is well-known, Cu permits an Ni plating layer to be easily formed thereon, whereas Mo is liable to oxidization and therefore a hard and brittle oxide film may occur on its surface, thus making it difficult to form an Ni plating layer.
For example, Patent Document 1 discloses a technique for suppressing defects and failures such as gaps and fissures at a bonding site between a heat spreader and a metal part. There, when bonding a heat spreader of a Cu—Mo composite alloy with an Mo metal part, the respective entire surfaces are subjected to separate Ni plating treatments to provide an improved wettability with the brazing material. However, this method requires separate Ni plating treatments to be performed which are suited to the respective materials, thus resulting in inferior productivity.
Alternatively, a method is generally used in which, for a Cu—Mo substrate, a pretreatment step is performed which involves etching the surface of the Mo substrate with red prussiate (potassium ferricyanide) before an Ni plating layer is formed by electroplating technique, and performing a diffusion heat treatment after depositing a thin Au film or a thin Ni film. However, according to this method, as will be described in connection with the Examples set forth below, a good Ni plating layer will be formed on the Mo substrate, but the Cu surface will become coarse and have bulges and the like through etching, thus causing the Ni plating layer to peel. Moreover, according to this method, many processes must be performed prior to Ni plating, thus resulting in a lower productivity.
On the other hand, Patent Document 2 describes a method in which an Ni plating layer is directly formed on the surface of a Cu—Mo substrate by using an electroless plating technique. As compared to electroplating, electroless plating has advantages of permitting uniform plating of a workpiece that has a complicated shape, and providing a coating of Ni plating which is high in hardness and excellent in abrasion resistance.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 6-344131 (Sumitomo Electric Industries, Ltd.)
[Patent Document 2] Japanese Laid-Open Patent Publication No. 62-183132 (Fuji Electric Co., Ltd.)